The occurrence of the cysticercoids ofAcanthocirrus retrirostris (Krabbe 1869) Baer 1956 (Cyclophyllidea,Dilepididae) and the metacercariae ofMaritrema gratiosum Nicoll 1907 (Digenea,Microphallidae) in the barnacle,Balanus balanoides (L.), on the coast of Yorkshire,England

A total of 1806 specimens of barnacles,Balanus balanoides, collected from the Yorkshire coast of England was examined for encysted helminth larvae. Twelve (0.7%) barnacles were infected with the hitherto unknown cysticercoid ofAcanthocirrus retrirostris (Krabbe, 1869) (Cyclophyllidea, Dilepididae). A total of 16 cysticercoids was found and their essential features are described. Also, 309 (17%) barnacles were infected with metacercariae ofMaritrema gratiosum Nicoll, 1907 (Digenea, Microphallidae). The mean intensity ofMaritrema gratiosum in infected barnacles was 1.4, and the frequency distribution of number of metacercarial cysts per barnacle agreed closely with a negative binomial frequency distribution. The ecological relationships of the cysticercoids and metacercariae with barnacles, the avian definitive hosts and the environment are discussed.